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Chlamydia is an infection that you can get when you have sex. If you have chlamydia, you probably won't feel sick and may not know you have it. That's why it's often called the "silent disease."

Chlamydia is an infection caused by bacteria (germs) called Chlamydia trachomatis. The bacteria can be passed from person to person during sex.

• If you're a man, it can affect the tube (urethra) that carries urine down from your bladder to the outside.
• If you're a woman, it can affect the neck of your womb (cervix) or your urethra, or both.
• The infection can be passed on by oral sex or anal sex as well as vaginal sex. So it can also affect your throat and your back passage (rectum).
• If you touch your eyes after touching infected fluid from your genitals you can get eye infection.¹

Newborn babies can also be affected by chlamydia. This is because women who have the infection can pass it on during childbirth. If you're pregnant and you have chlamydia, your baby may be born with eye infections or pneumonia. Both of these problems can be treated with antibiotics.²

Here we've looked at treatments for uncomplicated chlamydia. This is when the infection hasn't spread beyond your urethra if you're a man or the neck of your womb if you're a woman.³

You're most likely to get chlamydia if:⁴

• You're younger than 25
• You have a new sex partner or more than one sex partner
• You don't use condoms during sex.

If you're a woman, you are also more likely to get chlamydia if you're taking the contraceptive pill.

Chlamydia is sometimes called the "silent disease" because you can have it without knowing it. Up to 8 in 10 women with chlamydia and 5 in 10 men don't have any obvious signs of infection.² This is a problem for two reasons.

• People who don't know they are infected can keep on infecting other people.
• If it isn't diagnosed and treated, chlamydia can cause more serious health problems. (See What will happen to me?)

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.